1. Field of the Invention
The invention herein relates to catalytic polymerization methods and polymerization catalysts. More particularly, it relates to the polymerization of cyanoacrylate monomers.
2. Description of the Prior Art
The cyanoacrylate polymers are well known, especially for their adhesive properties. Their chemistry, methods of formation, compositions and uses are detailed widely; see, for example, Millet, "Cyanoacrylate Adhesives," Ch. 6, pp. 249-307, in Hartshorn, ed., Structural Adhesives: Chemistry and Technology (1986); Seymour, Engineering Polymer Sourcebook, Ch. 8, pp. 131-142 (1989); and Coover, "Cyanoacrylate Adhesives," Ch. 31, pp. 409-414, in Skeist, ed., Handbook of Adhesives (1962).
When used in the form of thin films, some cyanoacrylate monomers polymerize very rapidly in the presence of moisture. For these polymers, the amount of moisture in surrounding air is sufficient to cause complete polymerization of a thin film of the cyanoacrylate monomer, thus leading to the wide use of such cyanoacrylates as adhesives for bonding of tightly fitting surfaces. However, many of these polymers, when in the form of thin films, lack flexibility, color stability or film strength. Other cyanoacrylate monomers polymerize only slowly in the presence of ambient moisture. In addition, many cyanoacrylate monomers (including those that otherwise would polymerize rapidly in the presence of ambient moisture) are initially mixed with inhibitors intended to prolong shelf life, which has the detrimental side effect that desirable polymerization speed is substantially slowed. There is thus a need for a catalyst that would speed the thin film polymerization of the normally slow cyanoacrylate monomers, overcome the retarding effects of inhibitors mixed with other cyanoacrylate monomers, and provide flexibility, color stability and/or strength to thin cyanoacrylate polymer films.
Further, when present in a thicker layer (i.e., in "bulk"), most cyanoacrylate monomers do not polymerize well. Polymerization in bulk is usually incomplete and the presence of unreacted monomer is detrimental to the properties of the polymerized product. It is believed that the polymerization of the bulk material is incomplete because the ambient moisture which catalyzes the thin film polymerization cannot penetrate satisfactorily into the body of the thicker layers of the cyanoacrylate monomer, and even that amount of moisture which does penetrate does not act effectively as a catalyst.
This is of particular significance in the adhesive field. As noted above, the cyanoacrylates in thin films usually form excellent bonds with nylons, polystyrenes, polyesters, polyacetals, metals, glass and non-porous ceramics, woods and similar materials. However, in the thicker layers, such bonds do not form properly and the adhesion is weak or non-existent. Thus it has been difficult to produce "gap filling" cyanoacrylate adhesives, i.e., those adhesives which will bond tightly even where the opposing faces of the objects to be adhered do not mate precisely.
To this end there have been suggestions of a number of additive materials which can be incorporated into the cyanoacrylate monomer composition in an attempt to catalyze bulk polymerization fully and to provide for "gap filling." Some of the additives have been in the form of polymerization initiators but these are not usually satisfactory since they do not provide for shelf life prior to use. Another group of additives which has been suggested are promoters, which rely on separate initiation of polymerization by a third component. While such promoters can be blended with the cyanoacrylate monomers without causing any reaction, there is still the necessity of having a separate initiator before the polymerization promoters become effective. In addition, there have been compositions in which fillers are added to provide intermediate surfaces to help form extended films to "bridge the gap" between irregular mating opposed faces, but such fillers do not appreciably aid in the bonding and provide foreign material (and occasionally discoloration) to the bonded region.
While a number of the additives have been effective for completing or accelerating polymerization, they present problems which can prevent their full utilization. First, many of the catalytic reactions which result generate severe exotherms and cause the polymerized cyanoacrylate material to become overheated. The presence of such excess heat can deteriorate the polymer bonds, discolor the polymerized body and detrimentally affect other additive materials which may be present in the composition or the surfaces with which the cyanoacrylate polymer is in contact. For instance, when cyanoacrylate polymers are used as a surface coating for natural human fingernails or as part of nail preparation compositions which are used to enhance the physical properties and appearance of human fingernails, the exothermic reaction which occurs can not only discolor the decorative nail surface but can often be of sufficient magnitude that the person's nails and in some cases even the underlying tissue are seriously harmed by the heat. Of course even where there is no actual burning or blistering of the nails or tissue, the person usually experiences a great deal of discomfort from the heat. As a similar example, the bonding of transparent or translucent sheets of plastic materials is detrimentally affected if the exotherm is sufficient to scorch or discolor the bonded surfaces.
It has also been found that many of the catalyst compositions are toxic, either when inhaled as fumes or when placed in contact with the human skin. Again considering nail compositions, a volatile toxic catalyst can be inhaled by both the wearer of the nail compositions and by the beautician or other person who applies the compositions. Both also can absorb the material through the skin, either by directly touching the composition while it is being applied to the wearer's nails or by absorption of fumes through the various skin surfaces which are in contact with the ambient air. The same will of course be true for other user of the material, whether is it being used to catalyze cyanoacrylate adhesives or other surface coatings.
Finally, many of the proposed catalytic materials react only fairly slowly and do not provide quick polymerization of the slow or bulk cyanoacrylate monomers. This requires that such compositions must be set in place for some time before the cyanoacrylate monomer is fully polymerized. In many cases this is difficult or disadvantageous.
Organotin compounds have been disclosed in the prior art as catalysts for vinyl halide polymerization in combination with an oxidizing agent; see related U.S. Pat. Nos. 3,382,223 and 3,390,143. Polymerization was conducted under cryogenic conditions (-40.degree. C.) but at room temperature (+30.degree. C.) there was no polymerization reaction.
It would, therefore, be of great value to have a catalytic system which would allow for complete and rapid polymerization of cyanoacrylate monomers in bulk (i.e non-thin-film) form so that the polymerization would be complete throughout the polymerized body, that would accelerate the polymerization of "slow" thin film cyanoacrylate monomers, that could be used to overcome the retarding effects of inhibitors and that would provide flexibility, color stability and/or strength to thin cyanoacrylate polymer films. Such a system would also advantageously operate without the presence of any significant exotherm; would be reactive at ambient temperatures; would not tend to discolor or distort the polymerized body; and would not cause harm to any adjoining material, whether human tissue or nails or any other type of material with which the cyanoacrylate compositions may be in contact. Further, the catalyst itself should be readily available, satisfactory for use in either direct or indirect contact with people and of sufficiently low toxicity to be suitable for prolonged use by and in the presence of people, particularly where such compositions are to be applied directly to the human body (such as nail dressings), in which case they must also be acceptable for use under appropriate public health and cosmetics laws.